Key Technology Research Of NPC Three-level Shunt Active Power Filter

With the rapid development of China’s economy and the increasing of people’s living standards,the demands for power quality are getting higher and higher,and the problems of harmonic pollution in power distribution grid are drawn more and more attention.Active power filter(APF),as a high performance harmonic elimination device,can suppress the harmonic and purify the power grid effectively and quickly.Hence,APF provides a strong guarantee for the safe operation of the power system and the improvement of the quality of residential electricity.NPC(Neutral-Point Clamped)three-level APF is drawn much attention because of the characteristics of smaller output current ripple,higher DC-bus utilization and equivalent switching frequency.In this dissertation,some key technologies based on NPC three-level shunt APF,such as modulation strategy,neutral-point potential balancing algorithm and output current tracking technology in three-phase three-wire and four-wire power grid,is in-depth studied and some research results are achieved.A simplified SVPWM(Space Vector Pulse Width Modulation)strategy is proposed for NPC three-level APF.The sector of optimal vector,which normally needs complex transformation and translation of coordinate through traditional SVPWM,can be easily located by the polarity of three-phase reference voltage through the proposed strategy.Meanwhile,the duration time of three-phase reference voltage vectors can also be quickly calculated through voltage-second balance rule.Hence,the PWM signals can be generated easily to control the switching devices on or off.Moreover,the control objects of the modulation index extension and narrow pulse elimination can be conveniently achieved by modifying the duration time.The proposed SVPWM strategy with modulation index extension has not only the same DC-bus utilization as SVPWM,bus also reduces the computational burden of the processor.In order to solve the problem of neutral-point potential imbalance for three-phase three-wire APF,a novel neutral-point potential control strategy is proposed by analyzing the relationship among the current flowing through the neutral-point of DCbus,the three-phase output current,the deviation between upper and lower DC-bus voltage and three-phase duration time.The offset of neutral-point potential can be suppressed effectively by correcting the duration time directly without considering the selection of positive and negative small vectors.However,the reason of neutral-point potential offset for three-phase four-wire APF,due to the presence of the neutral-wire of power grid,is totally different from threephase three-wire APF.Therefore,the neutral-point balancing factor is firstly summed up by analyzing the relationship among the neutral-point potential offset,the amplitude and direction of phase current and the switching states.Secondly,a novel neutral-point potential control method is proposed through analyzing the different influence on neutral-point potential by three-phase switching states P,O and N.In this method,the duration time of state O,which will not affect neutral-point potential,is dispatched partially to states P and N which affect the neutral-point potential to achieve the target of suppressing the offset of neutral-point potential.Current control technology,as one of the key technologies,determines the compensation capability and accuracy of APF.In this dissertation,a novel predictive current control technology is proposed based on the analysis of several current control technologies.The proposed control technology,combining with the proposed simplified SVPWM strategy,can track the reference harmonic current quickly and accurately by optimizing the cost function.An APF experiment platform is designed independently in order to verify the modulation strategy and control methods proposed in this dissertation.The NPC threelevel topology is selected as the basic structure of the experiment platform and the architecture of DSP+FPGA is adopted as the core control part.In the last part of the dissertation some experiments were carried out for different harmonic loads and the experimental results verify the validity of the proposed modulation strategy and control method.